I'm hoping someone can confirm this. I want to install a tub/shower combo unit right on the slab, at ground level. I'm trying to figure out if I can do this. From what I understand, I'd have to dig down through the slab about 8 inches, to have room for the trap. I'd also need to slope the waste pipe about 1/4 inch per foot, from the top of the trap (which I assume can be pretty high, just below the floor of the tub?) to the main waste line of the house. Is this right? If not, please try to explain my error in really easy language, because I know very little about this stuff. Thanks!